Looking at some crash reports, it seems that an old version (perhaps the latest one too?) of the Yahoo Toolbar could be the cause.
"Yahoo! Toolbar {635abd67-4fe9-1b23-4f01-e679fa7484c1} 1.5.4.20081105 1.6.6.20090220"
At least, that's what I find that almost all of the crash reporters have in common. I've seen version 1.5.4 and 1.5.5 in the crashes, not sure if 1.6.6 solves the crashes or if people just haven't upgraded to it yet.

Created attachment 438711[details][diff][review]
patch
so, biesi explains:
this also points to the fact that mTransaction->Init failed btw
if it hadn't failed, SetupTransaction wouldn't call DeleteSelfOnConsumerThread
thus, this is simply a local bug.

A user reporting this crash signature on SUMO (with bp-52b6c13b-3cd2-44b3-98df-6b5cf2100720) said that disabling Yahoo Toolbar 1.6.5 stopped the crashing.
(This version of Yahoo Toolbar seems to be from 2008)

Thanks, biesi, I hope this can land soon as crashes rising to around #20 for a few days at times are surely something we're happy to go away.
It seems like this is only seen in higher volume in releases, so trunk and aurora don't show crashes from it yet due to their small user sample, 5.0 beta shows very few reports of it already, see https://crash-stats.mozilla.com/report/list?signature=nsHttpTransaction%3A%3ADeleteSelfOnConsumerThread()&version=Firefox%3A5.0 - I guess they'd only rise to significant level when we're shipping to the actual masses.
As the patch/fix looks _really_ simple and we should try to avoid that type of rising crashes on the upcoming releases, nominating for 5 and 6 (I guess approval requests should only come after trunk checkin and baking, right?).

Comment 22Smokey Ardisson (offline for a while; not following bugs - do not email)
2011-12-05 00:59:06 PST

Comment on attachment 438711[details][diff][review]
patch
This crash is our top crash for Camino 2.1--and causes certain users to crash repeatedly at startup--so we'd like approval to take the patch on 1.9.2 as well.
This bug was originally filed about 1.9.2 crashes, is marked "wanted" for 1.9.2, and has already baked on mozilla-central and various descendants since June.